Many types of optical discs include a data region and a label region. The data region is where the machine-readable digital data is written to, whereas the label region allows the user to record a human-readable label or image on the optical disc. A laser or another type of optical beam can be used to read from and/or write to the data region and the label region of an optical disc. For example, in the patent application entitled “Integrated CD/DVD Recording and Label”, filed on Oct. 11, 2001, and assigned Ser. No. 09/976,877, a type of optical disc is disclosed in which a laser or other optical beam can be used to write to the label region of an optical disc.
Optical discs that have optically writable label regions, such as those disclosed in the patent application assigned Ser. No. 09/976,877, may have a number of alignment marks preformed around inner or outer circumferences of the discs. These alignment marks are detected and used to determine the current angular position on an optical disc. Determining the current angular position on an optical disc is desirable, because it ensures that an image can be optically written to the optically writable label region of such an optical disc with high precision. However, the alignment marks on the optical disc occasionally may not be detectable. For instance, debris, dirt, or other foreign material may cover the alignment marks.
Even if just a small number of the alignment marks are not detectable, image quality may be severely degraded, or the image may be totally unreadable. If a large enough number of the alignment marks are not detectable, the drive may not even recognize the disk, making imaging impossible. Therefore, it is important for the drive to recognize the alignment marks and record their position correctly.